When you set an end date for your Campaign Dashboard, it will automatically archive at the end of that day. If you want to archive a Campaign Dashboard before that date, visit the settings section for the Dashboard (click on the gear button) and re-set the end date.
It's important to note that archiving a Campaign Dashboard does not freeze the data in it. If a tag, category, or author specified in a campaign is removed from a content item after archiving, the content item will drop from the archived campaign.
If you need to totally scratch a Campaign Dashboard and start over, go to the Campaign Dashboard settings page and scroll down to the bottom. There's a delete button in the lower right-hand corner.
Remember that you can't change any inputs—that is, tags, categories, and/or authors—after you've already created the Campaign Dashboard. If you need to change those, you'll have to delete the campaign and make a new one.
But, don't forget that you can always put in a date in the past for a new Campaign Dashboard, so you won't lose out on any data.